Add the "ward" classification to the unit. This hides the unit's command card and abilities.
Then create a trigger with
events: "a unit is issued a target order", "a unit is issued an immediate order", "a unit is issued an order targetting a point"
conditions: unit type = zombie and issued order = smart
action: issue order "stop".
You can still select the units and they are still your units, but you can't control them anymore. You can still give them orders via triggers, to create f.ex. a custom AI for those units.